Aortic stiffness measured in adolescence or adulthood determines current hypertension, predicts future incidence of hypertension, and future cardiovascular disease (CVD) events International hypertension guidelines list severe aortic stiffness as grounds to intensify anti-hypertensive pharmacotherapy. Mechanisms of arterial stiffness beyond aging and obesity warrant further elucidation. In our preliminary data from adolescents attending weight-loss summer camps arterial stiffness improvement was not associated with weight change but was with change in circulating carnitine. Carnitine influences fatty acid oxidation and carbohydrate metabolism. Carnitine could therefore link to arterial stiffness through insulin resistance which in turn affects cellular tone, vascular fibrosis, modification of lipids or glucose metabolism, and/or advanced glycation end products. This proposal leverages 2 instrumental variable study designs to infer a causal relation between carnitine and arterial stiffness. First, in 166 adolescents at risk of arterial stiffening due to high serum TGs, we will conduct a mechanistic, double blinded, RCT for the effect of 6 months of oral carnitine supplementation (CS+, n=83) versus placebo (CS-, n=83) on aortic stiffness measured as carotid femoral pulse wave velocity (CFPWV); serum fatty acid oxidation biomarkers by metabolomics analysis; insulin resistance as homeostatic model assessment of insulin resistance (HOMA-IR); and trigylcerides (TG). Aim 1 is to compare CS+ versus CS- on change in arterial stiffness and monitor adverse events. The hypothesis CS+ is associated with lower arterial stiffening, and CS+ effect is not modified by sex or race/ethnicity. Aim 2 is to compare the effect of CS+ versus CS- on fatty acid metabolism, insulin resistance, and lipids. The hypothesis is that CS+ alters long chain fatty acid beta oxidation, measured as lower long chain acylcarnitines, which in turn improves (HOMA-IR), and in turn decreases TG levels. This causal chain will be disentangled for direct versus indirect effects on CFWPV change. Second, naturally randomly assorted carnitine SNPs noted above will be used to characterize the relationship of carnitine to arterial stiffness and stratify the effectiveness of CS+.Aim 3a is to obtain the direct effect of carnitine on arterial stiffness using Mendelian randomization of SNPs associated with serum carnitine as instrumental variables with the hypothesis these variant SNPs are associated with lower arterial stiffness, supporting a causal inference. Aim 3b is to identify effect modification of CS+ vs CS- on arterial stiffness by examining if a carnitine genetic risk score will modify the effect of CS+ on change in arterial stiffness. This proposal with 2 instrumental variable projects would evaluate a causal role for carnitine in arterial stiffness at a point when the life course trajectory to hypertension can be modified. The study will also investigate the role of carnitine in insulin resistance and dyslipidemia at this same age, which may serve as grounds for future therapeutic clinical trials. Discovering genetically mediated causes of arterial stiffness or other outcomes may facilitate targeting of future therapies on susceptible youth before atherosclerotic changes are irreversible.